Calibration of multiple aperture ultrasound probes

ABSTRACT

The quality of ping-based ultrasound imaging is dependent on the accuracy of information describing the precise acoustic position of transmitting and receiving transducer elements. Improving the quality of transducer element position data can substantially improve the quality of ping-based ultrasound images, particularly those obtained using a multiple aperture ultrasound imaging probe, i.e., a probe with a total aperture greater than any anticipated maximum coherent aperture width. Various systems and methods for calibrating element position data for a probe are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/964,701, filed Aug. 12, 2013, which application claims the benefit ofU.S. Provisional Application No. 61/681,986, filed Aug. 10, 2012, titled“Calibration of Multiple Aperture Ultrasound Probes”, the contents ofwhich are incorporated by reference herein.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

FIELD

This disclosure generally relates to ultrasound imaging systems and moreparticularly to systems and methods for calibrating a multiple apertureultrasound probe.

BACKGROUND

In conventional ultrasonic imaging, a focused beam of ultrasound energyis transmitted into body tissues to be examined and the returned echoesare detected and plotted to form an image. While ultrasound has beenused extensively for diagnostic purposes, conventional ultrasound hasbeen greatly limited by depth of scanning, speckle noise, poor lateralresolution, obscured tissues and other such problems.

In order to insonify body tissues, an ultrasound beam is typicallyformed and focused either by a phased array or a shaped transducer.Phased array ultrasound is a commonly used method of steering andfocusing a narrow ultrasound beam for forming images in medicalultrasonography. A phased array probe has many small ultrasonictransducer elements, each of which can be pulsed individually. Byvarying the timing of ultrasound pulses (e.g., by pulsing elements oneby one in sequence along a row), a pattern of constructive interferenceis set up that results in a beam directed at a chosen angle. This isknown as beam steering. Such a steered ultrasound beam may then be sweptthrough the tissue or object being examined. Data from multiple beamsare then combined to make a visual image showing a slice through theobject.

Traditionally, the same transducer or array used for transmitting anultrasound beam is used to detect the returning echoes. This designconfiguration lies at the heart of one of the most significantlimitations in the use of ultrasonic imaging for medical purposes: poorlateral resolution. Theoretically, the lateral resolution could beimproved by increasing the width of the aperture of an ultrasonic probe,but practical problems involved with aperture size increase have keptapertures small. Unquestionably, ultrasonic imaging has been very usefuleven with this limitation, but it could be more effective with betterresolution.

SUMMARY OF THE DISCLOSURE

A method of calibrating an ultrasound probe is provided, comprising thesteps of placing a first array and a second array of the ultrasoundprobe in position to image a phantom, each of the first and secondarrays having a plurality of transducer elements, imaging the phantomwith the first array to obtain a reference image, wherein imaging isdependent on data describing a position of each transducer element ofthe first array, imaging the phantom with the second array to obtain atest image, wherein imaging is dependent on data describing a positionof each transducer element of the second array, quantifying a firsterror between the reference image and the test image; iterativelyoptimizing the data describing the position of each transducer elementof the second array until the first error is at a minimum.

In some embodiments, the method further comprises imaging the phantomwith a third array of the ultrasound probe to obtain a second testimage, the third array having a plurality of transducer elements,quantifying a second error between the reference image and the secondtest image and iteratively optimizing data describing a position of eachelement of the third array until the second error is minimized.

In some embodiments, the method further comprises storing raw echo datareceived while imaging the phantom with the second array.

In one embodiment, the iteratively optimizing step comprises adjustingthe data describing the position of the transducer elements of thesecond array to create first adjusted position data, re-beamforming thestored echo data using the first adjusted position data to form a secondtest image of the reflectors, quantifying a second error between thesecond test image and the reference image, and determining whether thesecond error is less than the first error.

In one embodiment, adjusting the data describing the position of thetransducer elements of the second array includes adjusting a position ofa reference point of the array and an angle of a surface of the array,but does not include adjusting a spacing between the elements of thesecond array.

In some embodiments, the method further comprises, after a firstiteratively optimizing step, performing a second iteratively optimizingstep comprising adjusting the first adjusted position data, includingadjusting a spacing between at least two transducer elements of thesecond array to create second adjusted position data, re-beamforming thestored echo data using the second adjusted position data to form a thirdtest image of the reflectors, quantifying a third error between thethird test image and the reference image, and determining whether thethird error is less than the second error.

In one embodiment, iteratively optimizing the transducer elementposition data comprises optimizing using a least squares optimizationprocess.

In other embodiments, quantifying the first error comprises quantifyinga distance between positions of reflectors in the reference imagerelative to positions of the same reflectors in the test image. In someembodiments, quantifying the first error comprises quantifying adifference in brightness between reflectors in the reference image andreflectors in the test image. In additional embodiments, quantifying thefirst error comprises quantifying a difference between a pattern ofreflectors and holes in the reference image compared with a pattern ofholes and reflectors in the test image.

In one embodiment, the reference image and the test image arethree-dimensional volumetric images of a three-dimensional pattern ofreflectors, holes, or both reflectors and holes.

In other embodiments, wherein the phantom comprises living tissue.

In some embodiments, the method further comprises identifying positionsof reflectors in the phantom and fitting a mathematically defined curveto a detected pattern of reflectors.

In one embodiment, the curve is a straight line.

In other embodiments, the step of quantifying a first error comprisescalculating a coefficient of determination that quantifies a degree offit of the curve to the pattern of reflectors.

A method of calibrating an ultrasound probe is provided, comprising thesteps of insonifying a plurality of reflectors of a phantom with theultrasound probe, receiving echo data with the ultrasound probe, storingthe echo data, beamforming the stored echo data using first transducerelement position data to form an image of the reflectors, obtainingreference data describing the reflectors, quantifying an error betweenthe image and the reference data, and iteratively optimizing thetransducer element position data based on the quantified error.

In some embodiments, the iteratively optimizing step comprisesiteratively optimizing the transducer element position data with a leastsquares optimization process.

In one embodiment, the iteratively optimizing step comprises adjustingthe transducer element position data, re-beamforming the stored echodata using the adjusted transducer element position data to form asecond image of the reflectors, quantifying a second error based on thesecond image, and evaluating the second error to determine whether theadjusted transducer element position data improves the image.

In some embodiments, adjusting the transducer element position datacomprises adjusting an array horizontal position variable, an arrayvertical position variable and an array angle variable. In otherembodiments, adjusting the transducer element position data does notcomprise adjusting a spacing between adjacent transducer elements on acommon array.

In one embodiment, the reference data is based on physical measurementsof the phantom.

In some embodiments, the method further comprises deriving the referencedata from a reference image of the phantom.

In one embodiment, the reference image is obtained using a differentgroup of transducer elements of the probe than a group of transducerelements used for the insonifying and receiving steps.

In additional embodiments, the step of iteratively optimizing thetransducer element position data comprises using a least squaresoptimization process.

In some embodiments, the method further comprises identifying positionsof reflectors in the phantom and fitting a mathematically defined curveto a detected pattern of reflectors. In one embodiment, the curve is astraight line.

In some embodiments, the step of quantifying a first error comprisescalculating a coefficient of determination that quantifies a degree offit of the curve to the pattern of reflectors.

A method of calibrating ultrasound imaging data is also provided,comprising the steps of retrieving raw echo data from a memory device,the raw echo data comprising a plurality of echo strings, each echostring comprising a collection of echo records corresponding to echoesof a single ultrasound ping transmitted from a single transmit apertureand received by a single receive element, retrieving first calibrationdata describing a position of each receive transducer elementcorresponding to each echo string, retrieving second calibration datadescribing a position of at least one transducer element correspondingto a transmitted ping associated with each echo string, forming areference image by beamforming a first collection of echo stringscorresponding to a first group of receive transducer elements, whereinbeamforming comprises triangulating a position of reflectors based onthe first and second calibration data, forming a test image bybeamforming a second collection of echo strings corresponding to asecond group of transducer elements that is not identical to the firstgroup of transducer elements, quantifying first error between thereference image and the test image, adjusting first calibration data todescribe adjusted positions for the elements of the second group,re-beamforming the test image with the adjusted positions for theelements of the second group to obtain a second test image, quantifyinga second error between the second test image and the reference image,and evaluating the new error to determine whether the second error isless than the first error.

In some embodiments, the method is performed without any physical orelectronic connection to a probe used to create the raw echo data.

In some embodiments, there is no ultrasound probe connected to thememory device.

An ultrasound probe calibration system is provided, comprising anultrasound probe having a plurality of transmit transducer elements anda plurality of receive transducer elements, a phantom having a patternof reflectors, a first memory device containing reference datadescribing the pattern of reflectors of the phantom, a second memorydevice containing transducer element position data describing a positionof each transmit transducer element and each receive transducer elementrelative to a common coordinate system, and an imaging control systemcontaining calibration program code configured to direct the system toinsonify the phantom with the transmit transducer elements, receive echodata with the receive transducer elements, and store echo data in athird memory device, form a first image of the pattern of reflectors bybeamforming the stored echo data using the transducer element positiondata, determine measurement data describing a position of the pattern ofreflectors as indicated by the first image, quantify an error betweenthe measurement data and the reference data, and iteratively optimizethe transducer element position data based on the quantified error.

In some embodiments, the imaging control system is configured toiteratively optimize the phantom by adjusting the transducer elementposition data; forming a second image of the pattern of reflectors byre-beamforming the stored echo data using the adjusted transducerelement position data quantifying a second error based on the secondimage and evaluating the second error to determine whether the adjustedtransducer element position data improves the image.

In one embodiment, the reference data is based on physical measurementsof the phantom.

In other embodiments, the reference data is based on a reference image.

In some embodiments, the imaging control system is configured toiteratively optimize the transducer element position data using a leastsquares optimization process.

In other embodiments, the phantom further comprises at least one regionthat absorbs ultrasound signals.

In some embodiments, the ultrasound probe comprises a plurality oftransducer arrays. In another embodiment, the ultrasound probe comprisesa single continuous transducer array. In one embodiment, the ultrasoundprobe comprises a transducer array with a concave curvature.

In some embodiments, the phantom comprises a pattern of pins.

In one embodiment, the phantom comprises living tissue.

In some embodiments, the calibration program code is configured todetermine measurement data by fitting a curve to a detected pattern ofreflectors.

In one embodiment, the calibration program code is configured toquantify an error by determining a coefficient of determinationquantifying a degree of fit of the curve.

In another embodiment, at least two of the first memory device, thesecond memory device, and the third memory device are logical portionsof a single physical memory device.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 is a schematic illustration of an embodiment of a three-apertureultrasound imaging probe and a phantom object being imaged.

FIG. 2 is a section view of one embodiment of a multiple apertureultrasound probe with a continuous curvilinear array positioned above aphantom and held in place by a clamp mechanism.

FIG. 3 is a section view of an embodiment of an adjustable multipleaperture imaging probe positioned above a phantom.

FIG. 4A is a longitudinal sectional view of a multiple apertureultrasound imaging probe configured for trans-esophageal ultrasoundimaging.

FIG. 4B is a longitudinal sectional view of a multiple apertureultrasound imaging probe configured for trans-rectal ultrasound imaging.

FIG. 4C is a longitudinal sectional view of a multiple apertureultrasound imaging probe configured for intravenous ultrasound.

FIG. 4D is a longitudinal sectional view of a multiple apertureultrasound imaging probe configured for trans-vaginal ultrasoundimaging.

FIG. 4E is a sectional view of a multiple aperture ultrasound imagingprobe configured for imaging round structures or features.

FIG. 4F is a plan view of a multiple aperture ultrasound imaging probewith a radial array of transducer elements configured forthree-dimensional imaging.

FIG. 5A is a cross-sectional view of an ultrasound probe calibrationphantom having a docking section with receiving slots for receiving andretaining ultrasound probes to be calibrated.

FIG. 5B is a top plan view of the ultrasound probe calibration phantomdocking section of FIG. 5A.

FIG. 6 is a process flow diagram of one embodiment of a process forcalibrating a multiple aperture ultrasound probe using a static phantom.

FIG. 7 is a process flow diagram illustrating one embodiment of aniterative optimization process for minimizing an error function byadjusting transducer element position variables.

FIG. 8 is a block diagram illustrating components of an ultrasoundimaging system in accordance with some embodiments.

DETAILED DESCRIPTION

The various embodiments will be described in detail with reference tothe accompanying drawings. References made to particular examples andimplementations are for illustrative purposes, and are not intended tolimit the scope of the invention or the claims.

The various embodiments herein provide systems and methods fordynamically calibrating a multiple aperture ultrasound probe using astatic phantom. Calibration of a multiple aperture ultrasound imagingprobe may generally comprise determining an acoustic position of eachtransducer element in the probe. Some embodiments of a dynamiccalibration process may generally include the steps of imaging acalibration phantom having a known pattern of reflectors, quantifying anerror between known information about the phantom and informationobtained from the imaging, and performing an iterative optimizationroutine to minimize an error function in order to obtain improvedtransducer element position variables. Such improved transducer elementposition variables may then be stored for use during subsequent imagingusing the calibrated probe.

Introduction & Definitions

Although the various embodiments are described herein with reference toultrasound imaging of various anatomic structures, it will be understoodthat many of the methods and devices shown and described herein may alsobe used in other applications, such as imaging and evaluatingnon-anatomic structures and objects. For example, the probes, systemsand methods described herein may be used in non-destructive testing orevaluation of various mechanical objects, structural objects ormaterials, such as welds, pipes, beams, plates, pressure vessels, etc.

As used herein the terms “ultrasound transducer” and “transducer” maycarry their ordinary meanings as understood by those skilled in the artof ultrasound imaging technologies, and may refer without limitation toany single component capable of converting an electrical signal into anultrasonic signal and/or vice versa. For example, in some embodiments,an ultrasound transducer may comprise a piezoelectric device. In otherembodiments, ultrasound transducers may comprise capacitivemicromachined ultrasound transducers (CMUT).

Transducers are often configured in arrays of multiple individualtransducer elements. As used herein, the terms “transducer array” or“array” generally refers to a collection of transducer elements mountedto a common backing plate. Such arrays may have one dimension (1D), twodimensions (2D), 1.X dimensions (1.XD) or three dimensions (3D). Otherdimensioned arrays as understood by those skilled in the art may also beused. Annular arrays, such as concentric circular arrays and ellipticalarrays may also be used. An element of a transducer array may be thesmallest discretely functional component of an array. For example, inthe case of an array of piezoelectric transducer elements, each elementmay be a single piezoelectric crystal or a single machined section of apiezoelectric crystal.

As used herein, the terms “transmit element” and “receive element” maycarry their ordinary meanings as understood by those skilled in the artof ultrasound imaging technologies. The term “transmit element” mayrefer without limitation to an ultrasound transducer element which atleast momentarily performs a transmit function in which an electricalsignal is converted into an ultrasound signal. Similarly, the term“receive element” may refer without limitation to an ultrasoundtransducer element which at least momentarily performs a receivefunction in which an ultrasound signal impinging on the element isconverted into an electrical signal. Transmission of ultrasound into amedium may also be referred to herein as “insonifying.” An object orstructure which reflects ultrasound waves may be referred to as a“reflector” or a “scatterer.”

As used herein, the term “aperture” may refer to a conceptual “opening”through which ultrasound signals may be sent and/or received. In actualpractice, an aperture is simply a single transducer element or a groupof transducer elements that are collectively managed as a common groupby imaging control electronics. For example, in some embodiments anaperture may be a physical grouping of elements which may be physicallyseparated from elements of an adjacent aperture. However, adjacentapertures need not necessarily be physically separated.

It should be noted that the terms “receive aperture,” “insonifyingaperture,” and/or “transmit aperture” are used herein to mean anindividual element, a group of elements within an array, or even entirearrays with in a common housing, that perform the desired transmit orreceive function from a desired physical viewpoint or aperture. In someembodiments, such transmit and receive apertures may be created asphysically separate components with dedicated functionality. In otherembodiments, any number of send and/or receive apertures may bedynamically defined electronically as needed. In other embodiments, amultiple aperture ultrasound imaging system may use a combination ofdedicated-function and dynamic-function apertures.

As used herein, the term “total aperture” refers to the total cumulativesize of all imaging apertures. In other words, the term “total aperture”may refer to one or more dimensions defined by a maximum distancebetween the furthest-most transducer elements of any combination of sendand/or receive elements used for a particular imaging cycle. Thus, thetotal aperture is made up of any number of sub-apertures designated assend or receive apertures for a particular cycle. In the case of asingle-aperture imaging arrangement, the total aperture, sub-aperture,transmit aperture, and receive aperture will all have the samedimensions. In the case of a multiple array probe, the dimensions of thetotal aperture may include the sum of the dimensions of all of thearrays.

In some embodiments, two apertures may be located adjacent one anotheron a continuous array. In still other embodiments, two apertures mayoverlap one another on a continuous array, such that at least oneelement functions as part of two separate apertures. The location,function, number of elements and physical size of an aperture may bedefined dynamically in any manner needed for a particular application.Constraints on these parameters for a particular application will bediscussed below and/or will be clear to the skilled artisan.

Elements and arrays described herein may also be multi-function. Thatis, the designation of transducer elements or arrays as transmitters inone instance does not preclude their immediate redesignation asreceivers in the next instance. Moreover, embodiments of the controlsystem herein include the capabilities for making such designationselectronically based on user inputs, pre-set scan or resolutioncriteria, or other automatically determined criteria.

As used herein the term “point source transmission” may refer to anintroduction of transmitted ultrasound energy into a medium from singlespatial location. This may be accomplished using a single ultrasoundtransducer element or combination of adjacent transducer elementstransmitting together as a single transmit aperture. A singletransmission from a point source transmit aperture approximates auniform spherical wave front, or in the case of imaging a 2D slice, auniform circular wave front within the 2D slice. In some cases, a singletransmission of a circular or spherical wave front from a point sourcetransmit aperture may be referred to herein as a “ping” or a “pointsource pulse.”

Point source transmission differs in its spatial characteristics from a“phased array transmission” which focuses energy in a particulardirection from the transducer element array. Phased array transmissionmanipulates the phase of a group of transducer elements in sequence soas to strengthen or steer an insonifying wave to a specific region ofinterest. A short duration phased array transmission may be referred toherein as a “phased array pulse.”

In some embodiments, multiple aperture imaging using a series oftransmitted pings may operate by transmitting a point-source ping from afirst transmit aperture and receiving echoes with elements of two ormore receive apertures, one or more of which may include some or allelements of a transmit aperture. A complete image may be formed bytriangulating the position of scatterers based on delay times betweenping transmission and reception of echoes, the speed of sound, and therelative positions of transmit and receive transducer elements. As aresult, each receive aperture may form a complete image from echoes ofeach transmitted ping. In some embodiments, a single time domain framemay be formed by combining images formed from echoes at two or morereceive apertures from a single transmitted ping. In other embodiments,a single time domain frame may be formed by combining images formed fromechoes received at one or more receive apertures from two or moretransmitted pings. In some such embodiments, the multiple transmittedpings may originate from different transmit apertures.

FIG. 1 illustrates an embodiment of a three-array multiple apertureultrasound imaging probe 10 and a phantom 20 to be imaged. The phantom20 generally includes a pattern of reflectors 30 within a solid orliquid medium 35. In some embodiments, a phantom 20 may also include oneor more “holes”—regions or objects that substantially absorb and do notreflect significant ultrasound signals. The probe 10 is shown with aleft transducer array 12 which may include three transmit apertureslabeled ‘n,’ ‘j,’ and ‘k’ (which may be referred to herein by short-handdesignations Ln, Lj and Lk). A right transducer array 14 may alsoinclude three transmit apertures ‘n,’ ‘j,’ and ‘k’ (which may bereferred to herein by short-hand designations Rn, Rj and Rk). Some orall of the elements of the left transducer array 12 may also bedesignated as a left receive aperture 13. Similarly, some or all of theelements of the right transducer array 14 may be designated as a rightreceive aperture 15. In addition to the left and right arrays, amultiple aperture ultrasound probe 10 may include a center transducerarray 16, which may include three transmit apertures labeled ‘n,’ ‘j,’and ‘k’ (which may be referred to herein by short-hand designations Cn,Cj and Ck). Some or all of the elements of the center transducer array16 may also be designated as a center receive aperture 17. It should beunderstood that each of the three apertures can include any number oftransducer elements which may be spaced from one another in one, two orthree dimensions.

In other embodiments, any other multiple aperture ultrasound imagingprobe may be calibrated using the systems and methods described below.For example, FIG. 2 illustrates a multiple aperture ultrasound probe 55with a single large (i.e., larger than an expected coherence width foran intended imaging application) continuous curved array 18 positionedover a phantom 20. Some embodiments of the calibration methods anddevices below may be particularly useful with adjustable probes such asthat illustrated in FIG. 3. FIG. 3 illustrates an adjustable multipleaperture ultrasound probe 11 positioned over a phantom 20. FIG. 4Aillustrates a multiple aperture ultrasound probe 100 with one or moretransducer arrays 102 positioned at a distal end of an endoscope 104sized and configured for transesophageal positioning and imaging. FIG.4B illustrates a multiple aperture ultrasound probe 110 with one or moretransducer arrays 112 and a housing 114 sized and configured fortrans-rectal positioning and imaging. FIG. 4C illustrates a multipleaperture ultrasound probe 120 including one or more transducer arrays122 and a housing 124 positioned at a distal end of a catheter 126 allof which may be sized and configured for intravenous positioning andimaging. FIG. 4D illustrates a multiple aperture ultrasound probe 130with one or more transducer arrays 132 and a housing 134 sized andconfigured for trans-vaginal positioning and imaging. FIG. 4Eillustrates a multiple aperture ultrasound probe 140 with a continuouscurved transducer array 142 and a housing 144 and a side-mounted cable146 sized and configured for positioning over curved anatomicalstructures such as arms and legs. FIG. 4F illustrates a multipleaperture ultrasound probe 150 with a large circular array 152 that mayhave a concave curvature about two axes. The probe of FIG. 4F and otherprobes may include transducer elements with substantial displacementalong orthogonal axes. Such probes may be particularly suitable fordirectly obtaining echo data from a three-dimensional volume. Any ofthese or other ultrasound probes (including single-aperture ultrasoundprobes) may be calibrated using the systems and methods herein.

As used herein, the term “phantom” may refer to any substantially staticobject to be imaged by an ultrasound probe. For example, any number ofphantoms designed for sonographer training are widely commerciallyavailable from various suppliers of medical equipment, such as Gammex,Inc. (gammex.com). Some commercially available phantoms are made tomimic the imaging characteristics of objects to be imaged such asspecific or generic human tissues. Such properties may or may not berequired by various embodiments of the invention as will be furtherdescribed below. The term “phantom” may also include other objects withsubstantially static reflectors, such as a region of a human or animalbody with substantially static strong reflectors. An object need not bepurpose-built as a phantom to be used as a phantom for the calibrationprocesses described herein.

With reference to FIG. 1, in one example embodiment of a multipleaperture imaging process, a first image may be formed by transmitting afirst ping from a first transmit aperture Ln and receiving echoes of thefirst ping at a left receive aperture 13. A second image may be formedfrom echoes of the first ping received at the right receive aperture 15.Third and fourth images may be formed by transmitting a second ping froma second transmit aperture Lj and receiving echoes of the second ping atthe left receive aperture 13 and the right receive aperture 15. In someembodiments, all four images may then be combined to form a single timedomain frame. In other embodiments, a single time domain frame may beobtained from echoes received at any number of receive apertures fromany number of pings transmitted by any number of transmit apertures.Time domain frames may then be displayed sequentially on a displayscreen as a continuous moving image. Still images may also be formedusing any of the above techniques.

In some embodiments, the width of a receive aperture may be limited bythe assumption that the speed of sound is the same for every path from ascatterer to each element of the receive aperture. In a narrow enoughreceive aperture this simplifying assumption is acceptable. However, asreceive aperture width increases, an inflection point is reached(referred to herein as the “maximum coherent aperture width” or“coherence width”) at which the echo return paths will necessarily passthough different types of tissue having different speeds of sound. Whenthis difference results in phase shifts in excess of 180 degrees,additional receive elements beyond the maximum coherent receive aperturewidth will actually degrade the image rather than improve it. Thecoherence width will vary depending on an intended imaging applicationand is difficult if not impossible to predict in advance.

Therefore, in order to make use of a wide probe with a total aperturewidth greater than the maximum coherent width, the full probe width maybe physically or logically divided into multiple apertures, each ofwhich may be limited to a width less than the maximum coherent aperturewidth and small enough to avoid phase cancellation of received signals.The maximum coherent width can be different for different patients andfor different probe positions on the same patient. In some embodiments,a compromise width may be determined for a given probe system. In otherembodiments, a multiple aperture ultrasound imaging control system maybe configured with a dynamic algorithm to subdivide the availableelements in multiple apertures into groups that are small enough toavoid significant phase cancellation.

In some embodiments, it may be difficult or impossible to meetadditional design constraints while grouping elements into apertureswith a width less than the maximum coherent width. For example, ifmaterial is too heterogeneous over very small areas, it may beimpractical to form apertures small enough to be less than the maximumcoherent width. Similarly, if a system is designed to image a very smalltarget at a substantial depth, an aperture with a width greater than themaximum coherent width may be needed. In such cases, a receive aperturewith a width greater than the maximum coherent width can be accommodatedby making additional adjustments or corrections may be made to accountfor differences in the speed-of-sound along different paths. Someexamples of such speed-of-sound adjustments are provided herein.

With a multiple aperture probe using a point-source transmission imagingtechnique (also referred to as ping-based imaging), each image pixel maybe assembled by beamforming received echo data to combine informationfrom echoes received at each of the multiple receive apertures and fromeach of the multiple transmit apertures. In some embodiments of multipleaperture imaging with point-source transmission, receive beamformingcomprises forming a pixel of a reconstructed image by summingtime-delayed echo returns on receive transducer elements from ascatterer in the object being examined. The time delays may bedetermined by the geometry of the probe elements and an assumed valuefor the speed of sound through the medium being imaged.

The locus of a single reflector will lie along an ellipse with a firstfocus at the position of the transmit transducer element(s) and thesecond focus at the position of the receive transducer element. Althoughseveral other possible reflectors lie along the same ellipse, echoes ofthe same reflector will also be received by each of the other receivetransducer elements of a receive aperture. The slightly differentpositions of each receive transducer element means that each receiveelement will define a slightly different ellipse for a given reflector.Accumulating the results by coherently summing the ellipses for allelements of a common receive aperture will indicate an intersection ofthe ellipses for a reflector, thereby converging towards a point atwhich to display a pixel representing the reflector. The echo amplitudesreceived by any number of receive elements may thereby be combined intoeach pixel value. In other embodiments the computation can be organizeddifferently to arrive at substantially the same image.

Because the position of each transmit and receive element plays animportant role in producing an image during ping-based ultrasoundimaging, the quality of an image produced from ping-based imaging issubstantially dependent on the accuracy of the information describingthe relative positions of the transducer elements.

Various algorithms may be used for combining echo signals received byseparate receive elements. For example, some embodiments may processecho-signals individually, plotting each echo signal at all possiblelocations along its ellipse, then proceeding to the next echo signal.Alternatively, each pixel location may be processed individually,identifying and processing all echoes potentially contributing to thatpixel location before proceeding to the next pixel location.

Image quality may be further improved by combining images formed by thebeamformer from one or more subsequent transmitted pings, transmittedfrom the same or a different point source (or multiple different pointsources). Still further improvements to image quality may be obtained bycombining images formed by more than one receive aperture. An importantconsideration is whether the summation of images from different pings,different transmit point-sources or different receive apertures shouldbe coherent summation (phase sensitive) or incoherent summation (summingmagnitude of the signals without phase information).

In some embodiments, multiple aperture imaging using a series oftransmitted pings may operate by transmitting a point-source ping from afirst transmit aperture and receiving echoes with elements of one ormore receive apertures (which may overlap with the transmit aperture). Acomplete image may be formed by triangulating the position of scatterersbased on delay times between transmission and receiving echoes and theknown position of each receive element relative to each point-sourcetransmit aperture. As a result, a complete image may be formed from datareceived at each receive aperture from echoes of each transmitted ping.

Images obtained from different unique combinations of a ping and areceive aperture may be referred to herein as image layers. Multipleimage layers may be combined to improve the overall quality of a finalcombined image. Thus, in some embodiments, the number of image layerscan be the product of the number of receive apertures and the number oftransmit apertures (where a “transmit aperture” can be a single transmitelement or a group of transmit elements). In other embodiments, the sameping imaging processes may also be performed using a single receiveaperture.

Phantom Calibration Embodiments

Some embodiments of ultrasound probe calibration methods using a phantommay generally include the steps of characterizing the phantom using someknown baseline reference data, then imaging the phantom with the probeto be calibrated. An error between the known reference data and dataobtained from the generated image may then be quantified and aniterative optimization routine may be used to obtain improved transducerelement position information. Such improved transducer element positionvariables may then be stored for use during subsequent imaging using thecalibrated probe.

FIG. 1 illustrates one embodiment of a phantom 20 that may be used forcalibrating a multiple aperture probe. In some embodiments, a phantom 20for calibrating a multiple aperture probe may include a plurality ofreflectors 30 arranged in a two-dimensional pattern within a solid,liquid or gel material 35 that has a consistent and knownspeed-of-sound. The reflectors may be made of any material, such as aplastic, metal, wood, ceramic, or any other solid material that issubstantially highly reflective of ultrasound waves relative to thesurrounding medium.

In some embodiments, reflectors 30 may be arranged in the phantom 20 ina pattern that may have characteristics selected to facilitate acalibration process. For example, a non-repeating reflector pattern willallow a calibration process to recognize an imaged position of thereflectors without confusion. For example, a complete grid pattern ishighly repetitive because portions of the pattern are identicallyduplicated merely by shifting one full grid position. In someembodiments, the pattern of reflectors may also comprise a number ofreflectors with displacement along the X axis 46 that is approximatelyequal to a number of reflectors with displacement along the Y axis 47.Thus, in some embodiments a pattern in the shape of a cross or a plussign may be used. In other embodiments, reflectors may be positionedrandomly or in other patterns, such as an X-shape, an asterisk, asunburst, a spiral or any other pattern.

In some embodiments, reflectors may also have depth or distinguishabledetail in the z-direction 48. For example, the reflectors 30 may be rodswith longitudinal axes along the z-direction 48. Alternatively, thereflectors may be substantially spherical or uniform three-dimensionalshapes. In other embodiments, an arrangement of intersecting wires orrods may be used to form a distinguishable pattern in three-dimensionalspace within a phantom.

The reflectors 30 in the calibration phantom 20 may be of any size orshape as desired. In some embodiments, the reflectors 30 may have acircular diameter that is on the same order of magnitude as thewavelength of the ultrasound signals being used. In general, smallerreflectors may provide better calibration, but in some embodiments theprecise size of the reflectors need not be an important factor. In someembodiments, all reflectors 30 in the phantom may be the same size asone another, while in other embodiments, reflectors 30 may be providedin a variety of sizes.

In some embodiments, the physical size and location of the reflectors inthe phantom 20 may be determined by mechanical measurement of thephantom (or by other methods, such as optical measurement or ultrasonicmeasurement using a known-calibrated system) prior to, during or afterconstruction of the phantom. Reflector position reference data may thenby obtained by storing the reflector location information within amemory device accessible by software or firmware performing acalibration process. Such reference data may include information such asthe position, size, orientation, arrangement or other information aboutthe reflectors and/or holes in the phantom. Reference data may berepresented or stored as a reference image or as a series of datapoints. Alternatively, reference data may be extracted from a referenceultrasound image.

In some embodiments, a reference image of the phantom may be obtainedusing a probe or an array within a probe that is known to bewell-calibrated. In other embodiments, a reference image of the phantommay be obtained using a selected group of elements of the probe.Reflector size and/or location information may then be determined fromthe reference image for use in calibrating remaining elements of theprobe or a different probe.

Therefore, in some embodiments a reference image may be obtained byretrieving previously-determined reflector position data from a memorydevice. In other embodiments, a reference image may be obtained byimaging the phantom using a sub-set of all elements in a probe. In someembodiments, it may be desirable to obtain a reference image using anaperture that is no wider than an assumed maximum coherence width (asdescribed above). This allows for a reference image to be formed withoutthe need to correct for speed-of-sound variations along differentultrasound wave paths. If the phantom is known to have a uniformspeed-of-sound (except for reflectors and/or holes), then the coherencewidth may be as large as an entire total aperture of a multiple apertureprobe. In such embodiments, obtaining a reference image with a receiveaperture smaller than the coherence width for an intended imagingapplication may be useful as a starting point.

For example, when calibrating a three-array probe such as that shown inFIG. 1, a reference image may be obtained by imaging the phantom 20using only one of the arrays (e.g., the center array 16, the left array12 or the right array 14). In other embodiments, such as whencalibrating a probe with a continuous convex transducer array 19 such asthat shown in FIG. 2, a reference image may be obtained by imaging thephantom 20 using only a small group of transducer elements of the array.For example, a group of elements near the center of the curved array maybe used as transmit and/or receive elements for obtaining a referenceimage. Similarly, a reference image may be obtained using a singleadjustable array 19 of an adjustable probe 11 such as that shown in FIG.3. Reference images may be obtained using any multiple apertureultrasound imaging probe in a similar manner.

As shown for example in FIG. 2, in some embodiments the phantom may bemounted in an enclosure that includes a probe-retaining portion 50. Amounting bracket 52 may also be provided to securely hold the probe 55in a consistent position relative to the phantom 20 during a calibrationprocess. Any mechanical bracket may be used. In some embodiments, acoupling gel and/or a gel or fluid-filled standoff 42 may be used toprovide a continuous medium through which the ultrasound signals willpass. The coupling gel and/or standoff 42 should have approximately thesame speed-of-sound as the phantom medium. In some embodiments, astandoff 42 may be a liquid or gel-filled bag.

FIG. 5A illustrates an alternative arrangement comprising a dockingsection 342 having a plurality of receiving slots 310 designed toreceive probes of specific shapes. The docking section 342 may be madeof the same material as the material of the phantom 20. Alternatively,the docking section 342 may be made of a material having the samespeed-of-sound characteristics as the phantom 20. As shown in FIG. 5B,many probe receiving slots 310 may be provided for a single dockingsection 342. In various embodiments, each probe receiving slot 310 maybe sized, shaped, and otherwise configured to receive one or morespecific ultrasound probes.

FIG. 6 is a process flow diagram illustrating an embodiment of a process400 for calibrating a multiple aperture probe using a phantom. Ingeneral, some embodiments of the process 400 may comprise the steps ofobtaining reference data 402 that characterizes known information aboutthe phantom (such as reflector or hole positions, sizes, etc.),insonifying the phantom with a test transmit (TX) aperture 404,receiving echoes with a test receive (RX) aperture 405, at leasttemporarily storing the received echo data 406, forming a test image ofthe reflectors by beamforming the echo data 408, determining an errorfunction 412 based on a comparison of the generated image and thereference data, and minimizing the error function 414 to obtain improvedtransducer element position variables 416. The resulting improvedelement position information may be stored in a memory device forsubsequent use by a beamforming process. Steps 404-416 may then berepeated for each additional transmit and/or aperture in the probe, andthe position of each transducer element in each transmit and/or receiveaperture within the probe may be determined relative to a commoncoordinate system.

In some embodiments, the process 400 may be entirely automated insoftware or firmware. In other embodiments, at least some steps mayinvolve human participation, such as to identify or to quantify an errorbetween an obtained image and a reference image. In other embodiments, ahuman user may also be called upon to determine whether a resultingimage is “good enough” or whether the calibration process should berepeated or continued.

In various embodiments, the process 400 may be used to calibrate theposition of one or more test transmit apertures, one or more testreceive apertures, or both. The choice of which type of aperture tocalibrate may depend on factors such as the construction of the probe,the number of transmit or receive apertures, or other factors. Thedefinitions of test transmit apertures and test receive apertures usedfor the calibration process may be, but need not necessarily be the sameas the definition of apertures used for normal imaging with the probe.Therefore, the phrase “test aperture” as used herein may refer to eithera transmit test aperture or a receive test aperture unless otherwisespecified.

In some embodiments, the test transmit aperture and the test receiveaperture used during the process 400 of FIG. 6 may be substantiallyclose to one another. For example, in some embodiments, the testtransmit aperture and the test receive aperture may be within anexpected coherence width of an intended imaging application relative toone another. For example, in some embodiments, a receive aperture mayinclude all elements on a common array (e.g., elements sharing a commonbacking block). Alternatively, a receive aperture may comprise elementsfrom two or more separate arrays. In further embodiments, a receiveaperture may include a selected group of transducer elements along alarge continuous array. In other embodiments, the test transmit apertureand the test receive aperture need not be close to one another, and maybe spaced from one another by a distance greater than any anticipatedcoherence width. In further embodiments, if the phantom is known to havea uniform speed of sound, the coherence width need not be a significantconsideration.

In some embodiments, a single transmit test aperture may be used toobtain both a reference image and data from which a test image may beformed. In such embodiments, a first receive aperture may be used toform a reference image, and a second (or third, etc.) receive aperturemay be used to form or obtain test image data. Similarly, a singlereceive aperture may be used for obtaining both a reference image anddata for a test image if different transmit apertures are used for thereference image and the test image data. Thus, the test transmitaperture and the test receive aperture need not necessarily be near oneanother. In other embodiments, reference images may be obtained usingtransmit and receive elements of a first array, while data for testimages may be obtained using transmit and receive elements of a secondarray, where the second array is a test array to be calibrated.

As described above, in some embodiments, the step of obtaining referencedata 402 may comprise retrieving reference data from a data storagedevice. Such a data storage device may be physically located within acalibration controller, within an ultrasound imaging system, within aprobe, or on a separate storage device that may be accessible via awired or wireless network connection. Alternatively, the step ofobtaining reference data 402 may comprise imaging the phantom with areference group of transducer elements.

In some embodiments, the step of insonifying the phantom with a testtransmit aperture 404 may comprise transmitting one or more pings fromone or more transmit elements of a transmit aperture. A single transmitaperture may typically comprise one, two, three or a small number ofadjacent elements.

After each transmitted ping, returning echoes may be received by allreceive elements of the test receive aperture, and the echo data may bedigitized and stored 406 in a digital memory device. The memory devicemay be any volatile or non-volatile digital memory device in anyphysical location that is electronically accessible by a computingdevice performing the imaging and calibration processes.

The received echo data may then be beamformed and processed to form atest image 408. In some embodiments, the steps of insonifying thephantom from a test transmit aperture 404 and receiving echoes with atest receive aperture 405 may be repeated using multiple combinations ofdifferent transmit apertures and/or receive apertures, and imagesobtained 408 from such transmitting and receiving may be combined in aprocess referred to as image layer combining prior to proceeding tosubsequent steps of the process 400.

In various embodiments, the error function may be determined from somedifference between the phantom reference data (e.g., information knownabout the position of reflectors in the phantom) and an image of thephantom obtained with the test receive aperture. In some embodiments,the choice of error function may be based on characteristics of thephantom used, available processing capabilities, a chosen optimizationmethod or many other factors.

In some embodiments, a modified least squares optimization method may beused to minimize an error function based on the square of an aggregatedstraight-line error distance between the expected reflector center andan imaged reflector center. For example, after forming an image of thephantom with the echoes received at a test receive aperture, the systemmay identify the location of each reflector in the image by identifyingthe brightest point in the image of approximately the expected size inapproximately the expected location of each known reflector. Once eachreflector is identified, an error between the imaged position and theexpected position of each reflector may be determined. In someembodiments, these individual reflector-position errors may then beaggregated into a collective reflector pattern error, such as by summingall individual reflector errors. Alternatively, the individual errorsmay be aggregated using any other function, such as taking a maximumerror, an average, or a weighted sum of individual errors. For example,if a phantom has some reflectors that are more difficult to detect thanothers, difficult-to-detect reflectors may be given less weight in theaggregate error function so as to obtain a more balanced result. Invarious embodiments, such individual and/or aggregate errors may beeither scalar or vector quantities.

In some embodiments, reflector images may be sought within apredetermined search area surrounding the expected location of eachreflector. The shape and size of a search area may be defined based onthe known pattern of reflectors and the distance between reflectors. Insome embodiments, images of reflectors may be identified by artificialintelligence or probability analysis using information about nearbyreflectors and the known pattern of reflectors. In other embodiments,the search area surrounding each reflector may comprise a circular,rectangular or other geometric area centered on the point of a center ofan expected reflector position. The size of a search area may beselected to be larger than the imaged reflectors, but typically smallenough that adjacent search areas do not overlap.

In some embodiments, when the actual positions of reflectors in thephantom are known, this knowledge may be used to greatly simplify theprocess of forming an image of the phantom. For example, forming animage 408 may be limited to beamforming only echoes representing searchareas surrounding the expected positions of reflectors in the phantom(rather than beamforming an entire image field). In other embodiments,beamforming may be limited to a search area defining the overall patternof reflectors. For example, this may be accomplished in some embodimentsby beamforming vertical and horizontal pixel bands slightly wider thanthe expected position of the pins in FIG. 1.

In some embodiments, the error function may be defined based on one ormore simplifying assumptions. For example, instead of detecting andoptimizing based on the two-dimensional or three-dimensional position ofeach individual reflector, a line or curve may be fit to the series ofreflectors. For example, using the phantom layout shown in FIG. 1, avertical line may be drawn through the pins spaced along the Y axis. Inpractice, reflectors in the approximate location of the vertical pinsmay be detected, a fit line through the detected reflectors may becalculated, and the quality of the fit line may be evaluated using afactor such as a coefficient of determination (R² value). An errorfunction may then be defined based on the R² value of the lineconnecting the vertical pins. A similar approach may be taken for thehorizontal pins. The simplifying assumption of pins fit to a line mayignore the spacing between the pins along the fit line, and maytherefore be less precise than methods defining an error function basedon two-dimensional position of each pin. However, optimizing based on asingle line segment may be substantially faster in processing terms thanoptimizing based a plurality of individual pin reflector positions.Therefore, such simplifications may still provide valuable informationin exchange for a faster processing time. In alternative embodiments,polynomial curves, circles or other mathematically-defined geometricshapes may be used as simplifications for representing a pattern ofreflectors within a phantom.

In other embodiments, the error function may be defined as some quantityother than reflector position. For example, in some embodiments, anerror function may be defined as a sum of absolute value differences inbrightness of the individual imaged reflectors relative to a referenceimage. In another embodiment, an error function may be defined based ona complete collective reflector pattern. For example, a phantom may bedesigned to contain an array of reflectors representing a referencenumber in binary form (i.e., a reflector may represent a ‘1’ and theabsence of a reflector at a grid position may represent a ‘0’). In suchembodiments, a calibration process may be configured to ‘read’ thebinary values, and the error function may be defined as the number ofbits different from the expected reference number. In furtherembodiments, an error function may be at least partially based on apattern of “holes”—regions of the phantom that absorb the ultrasoundenergy. Many other error functions may also be used.

FIG. 7 illustrates one embodiment of an iterative optimization process414 for minimizing an error function by adjusting transducer elementposition variables. After determining an initial error function (E₀) instep 412, the process 414 may proceed to iteratively seek a minimum ofan error function by making incremental adjustments to one or morevariables describing the position of the elements of the test transmitand/or receive aperture. Thus, during a first iteration, the process mayadjust 452 one or more initial test aperture element position variables(P₀) to obtain new test aperture element position variables (P₁).Without the need to re-insonify the phantom, the stored received echodata (from 406 in FIG. 6) may then be re-beamformed using the adjustedelement position parameters 454 (P₁) (image layers may also be combinedas needed during this step) in order to form a new image of the phantom.From the new image, a new error function (E₁) may be quantified 456 andthen evaluated or stored 460 before returning to step 452 for a seconditeration. The nature of the adjustments 452 and the error evaluations460 may depend on the type of optimization routine being used.

In some embodiments, adjustments to the element position variables maybe essentially random in each iteration (i.e., with no connection toadjustments made in prior iterations). Such random adjustments may bemade within a predetermined range of values relative to current elementposition data based on expectations of the possible degree ofmis-calibration of existing element position data. In the case of randomadjustments, an error function obtained from each iteration may bestored, and a minimum error function may be identified by comparing theresults of all iterations.

In other embodiments, adjustments may be directly based on informationfrom previous iterations, such as an evaluation of the magnitude and/ordirection of a change in the error value. For example, in someembodiments, if the new error function E₁ is less than the initial errorfunction E₀, then the adjustment made in step 452 may be determined tobe a good adjustment and the process may repeat for more iterationsmaking further incremental adjustments to the position variable(s). Ifthe new error function E₁ obtained in the first iteration is not lessthan the initial error function E₀ (i.e. E₁≧E₀), then it may be assumedthat the adjustment of step 452 was made in the wrong direction. Thus,in a second iteration, during step 452, the original element positionvariable(s) P₀ may be adjusted in a direction opposite to that triedduring the first iteration. If the resulting new error function E₂ isstill not smaller than the initial error function E₀, then the errorfunction is at a minimum (at least with respect to the adjusted elementposition variable(s)). In such a case, the error minimization processmay be stopped, and the last good position variables may be stored asthe new transducer element positions.

In some embodiments, the process 414 may be repeated through as manyiterations as needed until the error function is minimized. In otherembodiments, the process 414 may be stopped after a fixed number ofiterations. As will be clear to the skilled artisan, multiple ‘optimum’solutions may exist. As a result, in some embodiments, the iterativecalibration process may be repeated multiple times, and the results ofthe several calibrations may be compared (automatically using imageprocessing techniques or manually by a person) to identify a suitablesolution. In any event, it is not necessary to identify the absoluteoptimal result.

In various embodiments, the position of transducer elements may bedescribed by multiple variable quantities. Ultimately, it is desirableto know the acoustic position (which may be different than the element'sapparent mechanical position) of each transducer element relative tosome known coordinate system. Thus, in some embodiments, the acousticposition of each transducer element may be defined by an x, y, and zposition (e.g., with reference to a Cartesian coordinate system 45 suchas that shown in FIGS. 1-3). In adjusting such quantities during theoptimization process 414, position variables may be adjustedindividually or in groups.

Performing the optimization process by adjusting the x, y and z positionof each transducer element may be somewhat computationally intensive,since a single aperture may contain hundreds of individual elements.This may result in the iterative adjustment of several hundred if notthousands of variables. This is particularly true for probes with 2Darrays (i.e., those with transducer elements spaced from one another inX and Z directions), curved 1D or 2D arrays (i.e., arrays with curvatureabout either the X or the Z axis), and 3D arrays (i.e., probes withcurvature about two axes). While potentially computationally intensive,the various embodiments herein may be used to calibrate any ultrasoundprobe with large continuous planar or curved 1D or 2D arrays as well aslarge continuous 3D arrays with curvature about two axes.

As an alternative, some embodiments may employ one or more simplifyingassumptions. For example, in some embodiments it may be assumed thatelement position relationships within a single array remain fixedrelative to one another such that an array with a common backing blockwill only move, expand or contract uniformly. In some embodiments, itmay also be assumed that the elements are uniformly distributed acrossthe array. Using such assumptions, locating a center point of an array,a width of the array and an angle of the array surface relative to aknown datum may provide sufficient information about the acousticposition of each element. For example (with reference to FIG. 1), theposition of all elements in the left array 12 may be assumed based onoverall array position variables, which may include array width (‘w’),the position of the array's center (i) in the scan plane (i.e., the X-Yplane), and the angle of the array surface in the scan plane relative tosome baseline (θ). If it is assumed that the acoustic centers ofelements are uniformly distributed across the array with a consistentspacing in the X direction for a 1D array or in the X and Z directionsfor a 2D array, then the acoustic position of each transducer elementmay be mathematically expressed in terms of the above four variables(center-X, center-Y, width and angle). In some embodiments, if the arrayis a 2D array, a fifth variable describing the position of an array'scenter in the Z-direction (center-Z) may also be used. Alternatively,one or more of these variables may be treated as fixed in someembodiments. Using such simplifications, an error function minimizingprocess need only iteratively optimize four or five transducer elementposition variables. In the case of different probe constructions,different simplifying assumptions may also be used.

In some embodiments two or more optimization processes may be combinedin parallel or sequential processes in order to improve processingefficiency, calibration precision, or both. For example, in oneembodiment, a two-stage optimization process may be used in which afirst stage provides a coarse improvement to element position variableswhile relying on one or more simplifying assumptions. A second stage maythen provide a more detailed improvement to the element positionvariables while relying on fewer simplifying assumptions, but startingfrom the improved information obtained during the first stage. During afirst stage of one example embodiment, multiple reflectors may berepresented with a single geometric shape such as a line, and thespacing between transducer elements may be treated as fixed (i.e., suchvalues are not varied during the optimization). A second stage processmay then be performed, in which the position of each pin is optimized byvarying element position variables including the spacing betweentransducer elements.

In some embodiments, a similar calibration process may be used tocalibrate a probe 55 with a large continuous array 18, such as thatillustrated in FIG. 2. Because the continuous array 18 lacks physicalseparations, the same simplifying assumptions discussed above withregard to the probe of FIG. 1 may not apply. Instead, the probe 55 ofFIG. 2 may be calibrated by making simplifying assumptions about theshape of the large array, and apertures may be defined by usingrelatively small groups of elements at various positions along thearray. In some embodiments, the x-y position of each element in anaperture may be used as element position parameters to be optimized.Such selected apertures may then be calibrated in substantially the samemanner described above.

Regardless of the number of variables to be optimized in the iterativeerror function minimizing process 414, element position variables may beadjusted 452 either in series or in parallel. For example, inembodiments in which position variables are to be adjusted in series,only one variable may be adjusted during each iteration. In someembodiments of serial optimization, a single variable may be optimized(i.e., the error function may be minimized by adjusting only that singlevariable) before proceeding to the next variable. In embodiments inwhich two or more position variables are to be adjusted in parallel, thetwo or more variables may each be adjusted during each iteration. Insome embodiments, those two variables may be optimized before proceedingto optimization of other variables. Alternatively, all variables may beoptimized in parallel. In other embodiments, position variables may beoptimized using a combination of series and parallel approaches. Itshould be noted this distinction between series and paralleloptimization approaches should not be confused with parallel computerprocessing. Depending on computing hardware used, even optimizationsperformed in series as described above may be computed simultaneouslyusing separate threads in parallel processors.

After completing calibration of a first array or aperture, the processof FIG. 6 may be repeated for each remaining array or apertureindividually. For example, using the three-array probe of FIG. 1, thecalibration process may be repeated for the right array 14 and thenagain for the left array 12. After determining updated element positiondata for the first array, updated element position data for eachsubsequently-tested array may be determined and stored relative to acommon coordinate system such that the position of any element in theprobe may be determined relative to any other. For example, thecalibration process may determine the center of the center array, whichmay be used as the center of the coordinate system for the other arrays.The angle of the center array may also be used as a datum against whichangles of the other arrays may be defined. In other embodiments, thepositions and orientations of the apertures may be determined relativeto some other datum independent of any array. In other embodiments,element positions may ultimately be defined using any coordinate systemcentered around any point relative to the probe.

In some embodiments, transducer element position adjustments may beobtained and stored in the form of new corrected element positioncoordinates. In other embodiments, position adjustments may be obtainedand stored as coefficients to be added to or multiplied with previouselement position coordinates. For example, in some embodiments “factory”element position data may be stored in a read-only memory device in alocation readable by an ultrasound system, such as a ROM chip within aprobe housing. Such factory position data may be established at the timeof manufacturing the probe, and subsequent calibration data may bestored as coefficients that may be applied as adjustments to the factoryposition data.

In some embodiments, adjusted element position data for each transducerelement in a probe may be stored in a non-volatile memory device locatedwithin a probe housing. In other embodiments, adjusted element positiondata may be stored in a non-volatile memory device located within animaging system, on a remote server, or in any other location from whichthe information may be retrieved by an imaging system during imagebeamforming.

In some embodiments, a calibration process using the methods describedabove, may be particularly useful in rapidly re-calibrating anadjustable probe such as that illustrated in FIG. 3. Generally, an“adjustable probe” may be any ultrasound imaging probe in which theposition and/or orientation of one or more transducer arrays ortransducer elements may be changed relative to one or more othertransducer arrays or elements. Many adjustable probe configurationsbeyond that shown in FIG. 3 are possible and may be designed forspecific imaging applications.

In some embodiments, one or more of the arrays in an adjustable probemay be permanently secured to the housing in a fixed orientation andposition (e.g., the center array or the left or right end array), whilethe remaining arrays may be movable to conform to a shape of an objectto be imaged. The fixed array would then be in a permanently knownposition and orientation. Alternatively, the position and orientation ofone or more arrays may be known based on one or more position sensorswithin an adjustable probe. The known-position array(s) may then be usedto obtain a reference image of a phantom (or even a region of an objector patient to be imaged), and an optimization process may be used todetermine an adjusted position of the movable arrays. For example, asonographer may adjust the adjustable arrays of an adjustable probe toconform to a patient's anatomy. Then, during normal imaging, a referenceimage may be obtained using the known array, and positions of theremaining arrays may be determined by an optimization routine configuredto minimize an error function (e.g., using an optimization routine asdescribed above) defining an error between the reference image obtainedfrom the center array and images obtained from each adjustable array.

In other embodiments, a sonographer may adjust the arrays of anadjustable probe to conform to a patient's anatomy. The sonographer maythen place the probe onto a phantom that includes a conformable sectionconfigured to receive the probe in its adjusted position. For example, aconformable section may include a flexible bag containing a liquid orgel selected to transmit ultrasound signals at substantially the samespeed of sound as the material of the phantom. A calibration process maythen be initiated, and the position of each adjustable array may bedetermined by an iterative optimization routine in which reference datadescribing the phantom is compared with images of the phantom obtainedwith each array.

In some embodiments, the element-position information may change betweenperforming a calibration operation and capturing raw ultrasound data.For example, a probe may be dropped, damaged or may be otherwise altered(such as by thermal expansion or contraction due to a substantialtemperature change) before or during a raw sample data capture session.In some embodiments, the probe may be re-calibrated using captured,stored raw echo data as described below.

In other embodiments, a calibration system may be incorporated into anultrasound imaging system. In some embodiments, as shown for example inFIG. 8, an ultrasound imaging system 500 may include a raw data memorydevice 502 configured to capture and store raw, un-beamformed echo data.As shown in FIG. 8 an ultrasound imaging system configured to perform anoptimization-based calibration may include a transmit control subsystem504, a probe subsystem 506, a receive subsystem 508, an image generationsubsystem 510, a video subsystem 512, a calibration memory 530 and acalibration processor 540. The image generation subsystem may include abeamformer 520 (hardware or software) and an image-layer combining block522.

In some embodiments, a calibration system may be provided independentlyof an imaging system. In such embodiments, components such as the videosubsystem 512 may be omitted. Other components shown in FIG. 8 may alsobe omitted where practicable.

In practice, the transmit control subsystem 504 may direct the probe totransmit ultrasound signals into a phantom. Echoes returned to the probemay produce electrical signals which are fed into the receive sub-system508, processed by an analog front end, and converted into digital databy an analog-to-digital converter. The digitized echo data may then bestored in a raw data memory device 502. The digital echo data may thenbe processed by the beamformer 520 in order to determine the location ofeach reflector so as to form an image. In performing beamformingcalculations, the beamformer may retrieve calibration data from acalibration memory 530. The calibration data may describe the positionof each transducer element in the probe. In order to perform a newcalibration, the calibration processor may receive image data from theimage formation block 520 or from an image buffer memory device 526which may store single image frames and/or individual image layers.

The calibration processor may then perform an optimization-basedcalibration routine. Once a calibration process is complete, newcalibration information may be stored in the calibration memory device530 for use in subsequent imaging processes or in additional calibrationprocesses.

Using such a system, raw echo data of a phantom may be captured andstored along with raw echo data from a target object imaging session(e.g., with a patient). Capturing and storing raw echo data of a phantombefore and/or after an imaging session may allow for later optimizationof the imaging-session data. Such optimization may be applied at anypoint after the imaging session using the stored raw data and themethods described above.

As shown in FIG. 8, an ultrasound imaging system 500 may comprise anultrasound probe 506 which may include a plurality of individualultrasound transducer elements, some of which may be designated astransmit elements, and others of which may be designated as receiveelements. In some embodiments, each probe transducer element may convertultrasound vibrations into time-varying electrical signals and viceversa. In some embodiments, the probe 506 may include any number ofultrasound transducer arrays in any desired configuration. A probe 506used in connection with the systems and methods described herein may beof any configuration as desired, including single aperture and multipleaperture probes.

The transmission of ultrasound signals from elements of the probe 506may be controlled by a transmit controller 504. Upon receiving echoes oftransmit signals, the probe elements may generate time-varying electricsignals corresponding to the received ultrasound vibrations. Signalsrepresenting the received echoes may be output from the probe 506 andsent to a receive subsystem 508. In some embodiments, the receivesubsystem 508 may include multiple channels. Each channel may include ananalog front-end device (“AFE”) 509 and an analog-to-digital conversiondevice (ADC) 511. In some embodiments, each channel of the receivesubsystem 508 may also include digital filters and data conditioners(not shown) after the ADC 511. In some embodiments, analog filters priorto the ADC 511 may also be provided. The output of each ADC 511 may bedirected into a raw data memory device 502. In some embodiments, oneindependent channel of the receive subsystem 508 may be provided foreach receive transducer element of the probe 506. In other embodiments,two or more transducer elements may share a common receive channel.

In some embodiments, the ultrasound imaging system may store digitaldata representing the timing, phase, magnitude and/or the frequency ofultrasound echo signals received by each individual receive element in araw data memory device 502 before performing any further beamforming,filtering, image layer combining or other image processing.

In addition to received echo data, information about one or moreultrasound transmit signals that generated a particular set of echo datamay also be stored in a memory device, such as the raw data memorydevice 502 or another memory device. For example, when imaging with amultiple aperture ping ultrasound method as described above, it isdesirable to know information about a transmitted ping that produced aparticular set of echoes. Such information may include the identityand/or position of one or more a transmit elements as well as afrequency, magnitude, duration or other information describing atransmitted ultrasound signal. Transmit data is collectively referredherein to as “TX data”. In some embodiments, such TX data may be storedexplicitly in the same raw data memory device in which raw echo data isstored. For example, TX data describing a transmit signal may be storedas a header before or as a footer after a set of raw echo data generatedby the transmit signal. In other embodiments, TX data may be storedexplicitly in a separate memory device that is also accessible to asystem performing a beamforming process. In embodiments in whichtransmit data is stored explicitly, the phrases “raw echo data” or “rawdata” may also include such explicitly stored TX data.

TX data may also be stored implicitly. For example, if an imaging systemis configured to transmit consistently defined ultrasound signals (e.g.,consistent magnitude, shape, frequency, duration, etc.) in a consistentor known sequence, then such information may be assumed during abeamforming process. In such cases, the only information that needs tobe associated with the echo data is the position (or identity) of thetransmit transducer(s). In some embodiments, such information may beimplicitly obtained based on the organization of raw echo data in a rawdata memory.

For example, a system may be configured to store a fixed number of echorecords following each ping. In such embodiments, echoes from a firstping may be stored at memory positions 0 through ‘n’ (where ‘n’ is thenumber of records stored for each ping), and echoes from a second pingmay be stored at memory positions n+1 through 2n+1. In otherembodiments, one or more empty records may be left in between echo sets.In some embodiments received echo data may be stored using variousmemory interleaving techniques to imply a relationship between atransmitted ping and a received echo data point (or a group of echoes).In general, a collection of echo records corresponding to echoes of asingle transmitted ping received by a single receive element may bereferred to herein as a single “echo string.” A complete echo string mayrefer to all echoes of the single ping received by the receive element,whereas a partial string may refer to a sub-set of all echoes of thesingle ping received by the receive element.

Similarly, assuming data is sampled at a consistent, known samplingrate, the time at which each echo data point was received may beinferred from the position of that data point in memory. In someembodiments, the same techniques may also be used to implicitly storedata from multiple receive channels in a single raw data memory device.

In other embodiments, the raw echo data stored in the raw data memorydevice 520 may be in any other structure as desired, provided that asystem retrieving the echo data is able to determine which echo signalscorrespond to which receive transducer element and to which transmittedping. In some embodiments, position data describing the position of eachreceive transducer element may be stored in the calibration memorydevice along with information that may be linked to the echo datareceived by that same element. Similarly, position data describing theposition of each transmit transducer element may be stored in thecalibration memory device along with information that may be linked tothe TX data describing each transmitted ping.

In some embodiments, each echo string in the raw data memory device maybe associated with position data describing the position of the receivetransducer element that received the echoes and with data describing theposition of one or more transmit elements of a transmit aperture thattransmitted the ping that produced the echoes. Each echo string may alsobe associated with TX data describing characteristics of the transmittedping.

In some embodiments, a probe may be calibrated using raw echo datastored in a memory device without raw data of a phantom image. Assumingat least one array (or one portion of an array) is known or assumed tobe well-calibrated, nearly any image data with a pattern of strongreflectors may be used to calibrate second, third or further arrays orarray segments. For example, echo data from the known-calibratedaperture, array or array segment may be beamformed to obtain a referenceimage. Stored echo data from the remaining apertures/arrays may then becalibrated using any of the methods described above to calibrate theposition of the remaining arrays, apertures or array segments relativeto the first. By performing a calibration process using stored echodata, a probe may be calibrated even when neither the probe itself northe patient (or other imaged object) is physically present proximate tothe device performing the re-beamforming and image processing. In suchembodiments, the steps of insonifying a phantom 404, and receivingechoes 405 may be omitted from the process 400 of FIG. 6 at the time ofa calibration process, since those steps were performed during theimaging session in which the raw data was captured.

Although this invention has been disclosed in the context of certainpreferred embodiments and examples, it will be understood by thoseskilled in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. Various modifications to the above embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, it is intended that the scope ofthe present invention herein disclosed should not be limited by theparticular disclosed embodiments described above, but should bedetermined only by a fair reading of the claims that follow.

In particular, materials and manufacturing techniques may be employed aswithin the level of those with skill in the relevant art. Furthermore,reference to a singular item, includes the possibility that there areplural of the same items present. More specifically, as used herein andin the appended claims, the singular forms “a,” “and,” “said,” and “the”include plural referents unless the context clearly dictates otherwise.As used herein, unless explicitly stated otherwise, the term “or” isinclusive of all presented alternatives, and means essentially the sameas the commonly used phrase “and/or.” Thus, for example the phrase “A orB may be blue” may mean any of the following: A alone is blue, B aloneis blue, both A and B are blue, and A, B and C are blue. It is furthernoted that the claims may be drafted to exclude any optional element. Assuch, this statement is intended to serve as antecedent basis for use ofsuch exclusive terminology as “solely,” “only” and the like inconnection with the recitation of claim elements, or use of a “negative”limitation. Unless defined otherwise herein, all technical andscientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

What is claimed is:
 1. A method of calibrating an ultrasound probe,comprising the steps of: securing an ultrasound probe relative to anobject, the probe containing an array of ultrasound transducer elements;retrieving first transducer element position data defining a position ofeach transducer element of a first group of transducer elements relativeto a common coordinate system; imaging the object with the first groupof transducer elements to obtain a reference image, wherein with thefirst group of transducer elements imaging is dependent on the firsttransducer element position data; retrieving second transducer elementposition data defining a position of each transducer element of a secondgroup of transducer elements relative to the common coordinate system,the second group of elements including elements not within the firstgroup of elements; imaging the object with the second group oftransducer elements to obtain a test image, wherein imaging with thesecond group of transducer elements is dependent on the second positiondata; quantifying a first error between the reference image and the testimage; and iteratively optimizing the second transducer element positiondata until the first error is at a minimum.
 2. The method of claim 1,further comprising storing raw echo data received while imaging theobject with the second group of transducer elements.
 3. The method ofclaim 2, wherein the iteratively optimizing step comprises: adjustingthe second position data to create first adjusted position data;re-beamforming the stored echo data using the first adjusted positiondata to form a second test image of the reflectors; quantifying a seconderror between the second test image and the reference image; anddetermining whether the second error is less than the first error. 4.The method of claim 3, wherein adjusting the second position datacomprises adjusting one or more of an x position variable, a y positionvariable, and a z position variable for a plurality of elements of thesecond group.
 5. The method of claim 3, wherein adjusting the secondposition data describing the position of the transducer elements of thesecond group of transducer elements includes adjusting a position of areference point of the group of transducer elements and an angle of asurface of the group of transducer elements, but does not includeadjusting a spacing between the elements of the second array.
 6. Themethod of claim 5, further comprising, after a first iterativelyoptimizing step, performing a second iteratively optimizing stepcomprising: adjusting the first adjusted position data, includingadjusting a spacing between at least two transducer elements of thesecond group of transducer elements to create second adjusted positiondata; re-beamforming the stored echo data using the second adjustedposition data to form a third test image of the reflectors; quantifyinga third error between the third test image and the reference image; anddetermining whether the third error is less than the second error. 7.The method of claim 3, wherein adjusting the second position datacomprises changing one or more variables by a random quantity.
 8. Themethod of claim 3, further comprising, after determining whether thesecond error is less than the firs error, applying a second adjustmentto the second position data to create second adjusted position data,wherein a magnitude or a direction of the second adjustment is based onthe determination of whether the second error is less than the firsterror.
 9. The method of claim 1, wherein iteratively optimizing thesecond position data comprises optimizing using a least squaresoptimization process.
 10. The method of claim 1, wherein quantifying thefirst error comprises quantifying a distance between positions ofreflectors in the reference image relative to positions of the samereflectors in the test image.
 11. The method of claim 1, whereinquantifying the first error comprises quantifying a difference inbrightness between reflectors in the reference image and reflectors inthe test image.
 12. The method of claim 1, wherein quantifying the firsterror comprises quantifying a difference between a pattern of reflectorsand holes in the reference image compared with a pattern of holes andreflectors in the test image.
 13. The method of claim 1, wherein thereference image and the test image are three-dimensional volumetricimages of a three-dimensional pattern of reflectors, holes, or bothreflectors and holes.
 14. The method of claim 1, wherein the objectcomprises living tissue.
 15. The method of claim 1, further comprisingidentifying positions of reflectors in the object and fitting amathematically defined curve to a detected pattern of reflectors. 16.The method of claim 15, wherein the curve is a straight line.
 17. Themethod of claim 15, wherein the step of quantifying a first errorcomprises calculating a coefficient of determination that quantifies adegree of fit of the curve to the pattern of reflectors.